Preheat the oven to 350 degrees Fahrenheit. Cut the rhubarb into roughly 4-inch long pieces and toss them with 1 tablespoon of the olive oil and the sea salt. Spread them out evenly on a baking sheet and roast until softened, about 20 to 30 minutes depending on the thickness of the rhubarb stalk.
While the rhubarb is roasting, you can caramelize the spring onions. Heat the remaining 3 tablespoons olive oil in a large frying pan over medium low heat. Add the spring onions and stir to coat them in the olive oil. Reduce heat to low and continue cooking over low heat until the onions release their moisture and turn lightly golden, about 30 to 40 minutes, stirring every few minutes, increasing the stirring frequency towards the end of the coking time. Remove from heat and set aside.
To assemble, spread only one side of each slice of bread with the softened butter, using about 1/2 tablespoon per slice. Lay two slices of bread on a flat clean working surface with the buttered side facing down. Spread the exposed side of each slice with 1 teaspoon of honey, then sprinkle one slice with two or three tablespoons of the caramelized onion mixture and top with three or four pieces of the rhubarb and 2 ounces of the crumbled goat cheese. Place the other slice of bread, honeyed side facing down, on top of the goat cheese and press lightly to seal the sandwich together. Repeat with the remaining ingredients.
Melt the remaining tablespoon of butter in a large skillet over medium heat. Place two sandwiches next to each other in the skillet and cook until golden on each side and the cheese inside the sandwich is softened and melty, about 3 to 5 minutes per side. Repeat with the remaining sandwiches. Serve hot.